In the process of cleaning the surface of various substrates, for example, a semiconductor wafer (hereinafter referred to as wafer), is cleaned by applying chemicals to the surface thereof, and then rinsed with a processing solution such as pure water, and thereafter dried with an organic solvent such as isopropyl alcohol (hereinafter referred to as IPA). More specifically, the processing includes a process, in which, after it has been cleansed with chemicals and pure water, the wafer is exposed to IPA vapor to condense IPA on the wafer surface to substitute the IPA adhering to the wafer with pure water, and certain contaminants such as particles are washed out from the wafer surface, and a drying process by which IPA is vaporized to dry the wafer surface. In the drying process, even the smallest water droplet remaining on the wafer surface can form a water mark on the wafer surface, thereby adversely affecting the quality of the wafer. Therefore, in the semiconductor production process, it is necessary to ensure that contaminants do not adhere to the wafer. Various methods and apparatuses for processing substrate surfaces of wafers and the like, including measures dealing with contaminants, have been devised and put to practical use, and such method and apparatus for processing the substrate have been disclosed in several patent documents, including Japanese Laid-Open Patent No. 2001-271188 (see FIG. 1 and left column of page 5 to left column of page 6) and Japanese Laid-Open Patent No. H11-191549 (see claims, paragraph No. 0018 to paragraph [0024], and FIG. 1).
Hereafter, the substrate processing apparatus disclosed in Japanese Laid-Open Patent No. 2001-271188 will be described with reference to FIGS. 10 and 11. FIG. 10 is a sectional view of the substrate processing apparatus described in Japanese Laid-Open Patent No. 2001-271188. A substrate processing apparatus 1 comprising a processing vessel 2, a processing solution introducing pipe 3, a vapor generation unit 4, a processing solution drain unit 5, and heated-solvent supply devices 6 and 6′. The processing vessel 2 houses the substrates (for example, wafer) to be processed. The processing solution introducing pipe 3 supplies the processing solution (for example, pure water) into the processing vessel 2. The vapor generation unit 4 houses the organic solvent solution (for example, IPA). The processing solution drain unit 5 drains the processing solution from the processing vessel 2. The heated-solvent supply devices 6 and 6′ supply the heated organic solvent into the vapor generation unit 4. The plural substrates are conveyed into the processing vessel 2 while vertically standing in parallel at equal intervals, and substrate surface processing is performed.
In the substrate processing apparatus 1, the surface processing of various substrates, e.g., a semiconductor wafer W (hereinafter referred to as wafer W) is performed in accordance with the following processes.
1) Wafer Conveying Process
A lid 21 of the processing vessel 2 for keeping pure water J in a standby state in the processing apparatus 1 is opened, and the plural wafers W are conveyed to an inner vessel 22 of the processing vessel 2, and thrown and dipped into the pure water J, and the lid 21 is closed. Then, an inert gas such as a nitrogen gas is supplied from an inert gas supply pipe 81 into the processing vessel 2, to replace air in the processing vessel 2.
2) Drying Process
Then, after the wafer W is washed or rinsed, bubbling nitrogen gas N2 is supplied to the vapor generation unit 4 to generate vapor of an organic solvent solution, e.g., IPA, and the vapor generated is conveyed from the vapor discharge port 41 to the processing vessel 2, and the space above the pure water J therein is filled with the vapor.
Then, the on-off valve of an inner-vessel drain pipe 51 is opened to drain the pure water J in the inner vessel 22 bit by bit through a flow control valve, and the liquid level of the pure water J gradually decreases to expose the wafer W from the upper end thereof to the liquid surface of the pure water J.
When the surface of wafer W is exposed while lying above the liquid surface of the pure water J, the IPA vapor in the processing vessel 2 comes into contact with the surface of wafer W. At this point, because the pure water J of the processing vessel 2 is substantially set at room temperature, the temperature of the wafer W nearly reaches room temperature also. Therefore, by coming into contact with the wafer W, the IPA vapor is rapidly cooled and condenses on the surface of the wafer W on the liquid surface. The condensed IPA reduces surface tension of the pure water and acts to replace the pure water that has adhered to the wafer W. After the pure water J is completely drained, inert gas is supplied to the processing vessel 2 from the inter gas supply pipe 81 in order to dry the surface of the wafer W by causing the IPA to evaporate.
3) Wafer Taking Out Process
Then, after the pure water J is drained from the inner vessel 22, the substrate processing is completed by taking out the wafer W from the processing vessel 2 upon opening the lid 21.
In the substrate processing apparatus 1, since the series of processes is performed in one closed processing vessel, the wafer never comes into contact with the atmosphere, and the adhesion of contaminants such as foreign particles and water marks as well to the wafer surface can be avoided while ensuring efficient processing thereof.
However, in recent years, it has become necessary to insert into the processing vessel wafers processed by this kind of substrate processing apparatus in large numbers as may be possible in order to increase processing efficiency. In some cases, the wafers are simultaneously processed in the processing vessel in lot units ranging from 50 to 100 wafers, so that the space between wafers tends to become smaller. In addition, the diameter of wafers has increased from 200 mm to 300 mm. Accordingly, while the generation of water marks can be suppressed in wafers of relatively small diameter, such as 200 mm or less, such is not the case for wafers having a diameter of as large as 300 mm. The processing efficiency of the conventional apparatus is therefore limited.
By examining water marks formed from various angles, the inventors discovered that since the IPA vapor in dry gas is obtained by bubbling the inert gas in IPA, it contains a large amount of liquid particles of micro IPA (hereinafter referred to as “mist”) other than IPA gas with lower saturation concentration, and the size and mass of the mist is larger than those of nitrogen gas. Therefore, the IPA mist hardly passes through the narrow gaps between wafers. Accordingly, when the diameter of the wafer becomes 300 mm, IPA substitution is not sufficiently performed because the IPA mist is hardly supplied to the wafer surface which is far from a supply port of the IPA mist. In other wards, in cases involving the same total amount of IPA contained in the dry gas, the number of mists decreases as the size of the IPA mist increases. Conversely, the number of mists increases as the size of the IPA mist decreases. Further, when the IPA mist is large in size, the mass is heavy, thereby diminishing its moving speed. Therefore, in the drying process described in section (2) above, even if dry gas is supplied between the plural wafers in the processing vessel, there is an imbalance between the number of water droplets of the rinsing solution adhering to the wafer surface and the number of IPA mist particles. For example, when the number of IPA mist particles is smaller than the number of water droplets, IPA is not substituted for some of the water droplets which therefore remain, resulting in the generation of water marks.
In addition, since it is enormous in size and heavy, the IPA mist hardly passes through the narrow gaps between wafers. Therefore, in wafers having a diameter as large as 300 mm, almost all the IPA mists adhere to the wafer surface near the supply port of the IPA mist before the IPA mists reach the wafer surface which is relatively farther from the supply port. Accordingly, in the wafer surface distantly located from the supply port, the number of IPA mists supplied becomes smaller, and there is no even supply of IPA mists. The number of IPA mists supplied to the wafer surface near the supply port of the IPA mist is more than required while the number of IPA mists supplied to the wafer surface distantly located from the supply port is insufficient, resulting in the situation where IPA is not sufficiently substituted for the rinsing solution adhering to that part of the wafer surface which is distantly located from the IPA mist supply port, thereby resulting in the generation of water marks.
The state in which IPA is substituted for water droplets will be described with reference to FIG. 11. FIG. 11 is a sectional view schematically showing the relationship between the IPA mist and water droplets of the rinsing solution (hereinafter referred to as DIW) adhering onto the wafer W in the drying process. In the drying process described in section (2) above, as shown in FIG. 11(a), a mixture of a nitrogen gas N2 and the IPA vapor containing large-size IPA mist (liquid) is supplied into the processing vessel, for application to the spaces between wafers W. As shown in FIG. 11(b), although the IPA vapor is intended to replace DIW, the moving speed is slow due to the large size of the IPA mist, and because many wafers (50 to 100 wafers) with a diameter of as large as 300 mm are simultaneously processed in the processing vessel, the number of IPA mists is likewise limited, and thus there are occasions when the IPA mist does not reach all the DIWs. Thus, the IPA mist adheres to that portion of the wafer surface near the dry gas supply port before it reaches that portion of the wafer surface which is farther from the dry gas supply port, and thus no IPA mist is supplied to the wafer surface distantly located from the dry gas supply port. Accordingly, as shown in FIG. 11(c), DIW remains on the wafer surface, which results in the generation of water marks.
In the substrate processing apparatus disclosed in Japanese Laid-Open Patent No. H11-191549, the organic solvent is heated and evaporated to generate the mixture of organic solvent vapor and inert gas in the evaporation vessel without bubbling the inert gas in the organic solvent, and then the mixed gas is heated and kept constant while it is diluted with another inert gas originating from a piping, and the mixed gas is then emitted through a jet nozzle. In the above-mentioned substrate processing apparatus, the organic solvent vapor originating and emitted from the piping and the nozzle completely becomes the mixed gas to be used. In such event, because the size of the gaseous organic solvent molecule is much smaller than that of the mist, the problem related to the generation of water marks due to the use of the organic solvent mist does not arise.
However, even if the substrate processing is performed with the organic solvent vapor which completely becomes the mixed gas, because the concentration of organic solvent vapor in the dry gas does not go beyond the saturation point, the absolute amount of organic solvent in the dry gas is rather small. Accordingly, the spread of the organic solvent vapor to every corner of the large substrate to replace the moisture of the substrate surface takes considerable time and therefore the substrate processing apparatus disclosed in Japanese Laid-Open Patent No. H11-191549 does not address the need to decrease and substantially eliminate the number of water marks formed on the substrate surface while increasing the speed of carrying out the drying processing.
Although the term “vapor” generally means “gas” in the technical field of substrate processing, “vapor” can also refer to a gas containing “micro liquid particles (mist)”, such as the dry gas previously referred to. Accordingly, both meanings shall apply when “vapor” is mentioned in the present description and the claims.